This invention relates generally to non-reciprocal radio frequency devices and more particularly to the construction of circulators and isolators employing printed circuit microstrip resonators and ferrite disposed in layers of a multi-layer printed circuit laminate. Reference is made to copending patent application number 880,220 "Circulator/Isolator Resonator" filed on behalf of Robert C. Kane et al. on the same date as the present invention and containing related subject matter.
Circulators and isolators are well known radio frequency devices which exhibit non-reciprocal properties. A common form of circulator is that of a three-port device which couples a radio frequency signal from a first port to a second port with virtually no attenuation of the signal but which significantly attenuates a signal coupled from the second port to the first port. The signal applied at the second port is coupled to a third port. Likewise, a signal applied at the third port is coupled to the first port but greatly attenuated at the second port. Thus, the effect of the circulator is to isolate the ports from each other in one direction (ie. ports 1-3-2-1) and to couple the ports sequentially to each other in the other direction (ports 1-2-3-1).
Physical construction of circulators can be realized in resonant structures such as radio frequency resonant cavities and in waveguide at higher frequencies. Circulators may also be realized in planar configuration using stripline or microstrip technology which employ a planar resonating element between two ground plane conductors (stripline) or coupled to a single ground plane conductor (microstrip).
A microstrip circulator is typically constructed of an essentially planar resonating element having regularly spaced ports coupled to the periphery of the resonating element (resonator). The resonator is closely coupled to one or more elements exhibiting gyromagnetic properties (typically a ferrite material) and a magnetic field is applied perpendicular to the resonator and through the ferrite. When a radio frequency signal is applied at one port, the ferrite atoms, which have their magnetic spin vectors preferentially aligned along the magnetic lines, precess in one rotational direction. This precession favors coupling of the radio frequency signal from the port of application to the next port in the direction of the spin precession.
The preferential coupling of radio frequency signal from one port to another may be employed in a more particular application of a circulator known as an isolator. An isolator has one of the ports terminated in a resistance which matches the characteristic impedance of the circulator. This device permits radio frequency signal to propagate in one direction but blocks the propagation in the opposite direction (because the radio frequency signal energy is dissipated in the terminating resistance). This device also presents a relatively constant impedance to both the input and the output ports. Consequently, isolators are used extensively in radio transmitters and receivers where the impedance presented to an amplifying stage must be maintained close to the input or output impedance of the stage or where reflected energy could produce undesired effects or damage.
The quality of circulators and isolators is generally measured by the insertion loss in the direction of signal coupling, the amount of attenuation in the opposite direction, the impedance of each port, and the band of frequencies over which these characteristics are maintained. Optimization of these characteristics is the goal of circulator designers and has resulted in increasingly complex and expensive techniques of aligning the resonator, the ferrite, and the magnet. Additionally, improvements in cost and complexity of circulators has been realized by employing microstrip technologies rather than stripline technologies. Since microstrips require only a single groundplane and a majority of the propagating electromagnetic field is contained between the single conductor and the groundplane, only one ferrite and associated hardware is necessary to non-reciprocally transfer an applied radio frequency signal from one port of the circulator to another. Practical realizations of microstrip circulators, however, have produced circulators which require more volume than comparable stripline circulators.